Odor in Master Bathroom

DDYAZJune 14, 2011

We have a puzzling odor in our master bath. The odor only occurs in the evening, and when it occurs it is almost always between 8:30 and 10:00 PM. We have never experienced the odor during the day and the odor does not appear to be related to our showering/bathing schedules, etc. The house is a single story on a concrete slab in AZ. The bath has two vanity sinks, a tub, separate shower, and stool. So far I have ensured all the traps are wet, I have sealed off all the drains and overflows with duct tape or left water in the sinks, I have checked the plumbing vent on the roof for obstruction, I have crawled into the attic to check for something dead. I have inspected my exhaust fan ducting to look for a birds nest, and I even sealed off the exhaust fan housing on the off-chance that my AC units may be creating a negative pressure in the house allowing sewer gas from the plumbing vent on the roof to sneak back through my exhaust vent. All of the above has been to no avail. Has anyone got any ideas that I have overlooked, or suggestions on resources to help find the culprit. I'm out of ideas, and my wife has moved into the guest bedroom.

We've had similar experiences in several homes, but our findings/solutions won't necessarily be right for you.

In house one, the problem was found to be an errant screw. The screw was from a light fixture installation over a sink, which punctured the vent stack, allowing sewer gas to escape. The builder tried at least half a dozen different fixes (including tearing up our shower) before considering that.

In two other homes, a definitive cause was never found. We had air vents cleaned, sprayed them with disinfectant, etc., after checking everything plumbing related. One builder even paid to have a plumber run a camera scope through the pipes to see if there was hidden damage (that house was also on slab.) Nothing. I bought Roto-Rooter's Pipe Shield and began using it weekly in the master bathroom. The odor disappeared, but I cannot swear that the Pipe Shield did the trick.

At our current house, a plumber suggested that I pour a little mineral oil into each drain, especially the jacuzzi tub that we rarely used. He said it would help prevent the water in the trap from evaporating quickly in dry weather (or when the A/C ran non-stop in the summer.) I now use Biokleen Bac-Out in all drains once a month, too.

Thanks for the replies. The house is 12 years old, but this is not the first time we have had the problem. Previously we thought we were negligent in keeping the traps wet in a seldom used tub and shower, although I could never prove this conclusively, and the odor always eventually disappeared. This time the problem is much more persistent and obnoxious. Unfortunately getting to the vent pipe in the attic is a real problem because of the low ceiling/roof clearance in this alcove area and the maze of ductwork restricting access. Would a camera inspection inside the vent pipe be a suggestion for checking the joint connections, or could a pressure check be done on the vent plumbing? The mineral oil in the traps and the Bioklean suggestiona are good ones and we will try them. Maybe the evaporation issue is the culprit because we only have the problem in the summer when the AC is running, but I am still really confused by why the odor ONLY occurs in the evening. Thanks.

We have two furnace/AC units. Gas forced air furnaces with AC using same ductwork. The blowers are dual speed with the higher speed mostly used for cooling. As mentioned in earlier post we have never experienced the odor except during the summer. However, the odor has occurred when the AC has not been in use and no blowers were on.

Thanks for the reply. This is not a rotten egg smell as would be the case with a natural gas leak, and the gas line and regulator are on the far side of the house from this bathroom area. I'm pretty convinced this is a plumbing vent problem, but am puzzled by its "scheduled" occurences. Only appearing in the evening. I asked in an earlier post whether it is possible to do a pressure check on the vent lines but no-one has responded to this question. Any other suggestions out there?

Pressure testing is typically done before plumbing fixtures are set in place. In order to test an existing system, I believe you would have to pull each fixture and place a test plug at each opening, then you can test the entire system. I don't know of any other way.

"Pressure testing is typically done before plumbing fixtures are set in place. In order to test an existing system, I believe you would have to pull each fixture and place a test plug at each opening, then you can test the entire system. I don't know of any other way."

Plus, you would have to somehow plug the connection to the sewer. Typically, pressure testing is done before the permit to connect to the sewer is approved. I don't know, maybe an inflatable bladder on a line snake, sent down past the last wye connection to the main sewer line. I don't think it is a good idea to separate the pipes.

Thanks for the replies on the pressure check question. I currently have all the drains and overflows blocked off in this bathroom. I also have the exhaust vent sealed off in case gas from the plumbing vent is finding its way back inside through the near-by exhaust vent. These steps have almost eliminated the odor in the house. We had one evening with a slight smell inside, plus a stronger (same) smell outside the house. I'm giving it a few more days and then I will start un-blocking things one-at-a-time to see if I can locate the source.

Just a thought...considering the consistency of time of day which also corresponds with cooling of the air. Is it possible the cooler and heavier air is causing the vented gases to drop and follow the roof line and find it's way back into the house, perhaps through the vent or eaves? You said you had the vent sealed off, how about sealing from the outside too. Also, temporarily extend the sewer vent higher on the roof.

How about a neighbors proximity, could their vent gases be heading your way? If you live near or down wind from the sewage treatment plant or pumping station, it's odors cold drop lower in the evening and drift your way. This happens a lot in areas that do not properly pump and treat their sewage.

Perhaps the odor is getting into the house in a diluted manner and the chimney effect of the bathroom exhaust vent draws it into the bathroom in a more concentrated manner.

If you use gas, it's possible you are getting a down draft through the exhaust chimney from the cooler air and it concentrates into the bathroom.

What effect does the bathroom fan have on eliminating the odor and if it does, is it a long term effect? The exhaust gases from burning LP does have a similar odor as sewage gas. Also check your toilet to see if it fits loose to the flange, allowing gases to escape into the room. Check your drain pipe, at and anywhere after the traps under the sink and tub also.

Just trying to determine if the problem is indeed in the bathroom and not somewhere else and then concentrating in the bathroom.

Thanks maryland irisman. You're on the same trail I'm on. I strongly suspect we're getting a back-draft effect down through the exhaust vent. When I disassembled the exhaust fan housing a couple weeks ago I noticed the "el-cheapo" plastic flapper didn't seem to be closing properly. That's what led me to seal off this exhaust vent. Also, when I went up on the roof to inspect the vents for bird nests, etc., I did notice the odor right at the eave just below where the plumbing vent exits the roof. This plumbing vent is about 2 feet further up the roof from my exhaust vent....so if the lower evening temp could be allowing sewer gas to drop down toward the exhaust vent housing maybe that's where the odor is getting into the house. I am about ready to start unsealing my drains, overflows, etc. Once I am able to determine the exhaust vent is the culprit I will experiment with raising the plumbing vent stack. Thanks for your help, and I'll post the results.

The el-cheapo plastic flapper might just be letting the smell back in. Yes, I'd like to know what the final solution is, this topic comes up every year or so, it would be good to reference people back to this post.